Nanometer size inorganic materials exhibit a wide range of electrical and optical properties that depend on composition, size, shape, and surface ligands and are of both fundamental and technological interest as disclosed in Yin, Y. et al., A.P. Colloidal nanocrystal Synthesis and the organic-inorganic interface. Nature 437, 664-670 (2005) [1], in Hu, J. T. et al., C.M. Chemistry and physics in one dimension: Synthesis and properties of nanowires and nanotubes. Accounts Chem Res 32, 435-445 (1999) [2] and in Geim, A. K. et al., The rise of graphene. Nature Materials 6, 183-191 (2007) [3].
Well documented procedures to grow zero dimensional systems as disclosed in Murray, C. B. et al., M.G. Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites. J Am Chem Soc 115, 8706-8715 (1993) [4], dots, and one dimensional systems as disclosed in Duan, X. F. et al., C.M. General synthesis of compound semiconductor nanowires. Adv Mater 12, 298-302 (2000) [5] and Peng, X. G. et al. Shape control of CdSe nanocrystals. Nature 404, 59-61 (2000) [6], wires and tubes, as colloidal particles in solution have been reported.
In contrast, there are no methods of preparation that yield optically active two dimensional soluble particles.
Yet, ultra-thin films (quantum wells) of II-VI and III-V semiconductors epitaxially grown on substrates by molecular beam epitaxy for example have proven extremely useful for both fundamental studies and a wealth of applications in optoelectronics as disclosed in Weisbuch, C. et al. Quantum Semiconductor Structures: fundamentals and applications. (Academie Press, 1991) [7].
Wires, 2D for films—can be grown in gas phase syntheses on a substrate by molecular beam epitaxy (MBE) and other techniques as disclosed in [7] or from melted clusters by vapor-liquid-solid process as disclosed in Morales, A. M. et al., C.M. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 279, 208-211 (1998) [8]. They can also be grown in liquid phase colloidal synthesis in aqueous or non-hydrolytic media as disclosed in Jun, Y. W. et al., J. Shape control of semiconductor and metal oxide nanocrystals through nonhydrolytic colloidal routes. Angew Chemlnt Edit 45, 3414-3439 (2006) [9] As for the gas phase approaches, the non hydrolytic liquid phase synthesis, gives access to OD and 1D crystals with controlled nanometric size and shape as disclosed in reference [1], with the advantages that the crystals can be processed more easily for surface chemistry modification as disclosed in Michalet, X. et al., Quantum dots for live cells, in vivo imaging, and diagnostics. Science 307, 538-544 (2005) [10], core/shell synthesis as disclosed in Hines, M. A. et al., P. Synthesis and characterization of strongly luminescing ZnS-Capped CdSe nanocrystals. J Phys Chem-Us 100, 468-471 (1996) [11], directed assembly as disclosed in Redl, F. X. et al., S. Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots. Nature 423, 968-971 (2003) [12] or incorporation in polymer matrices or nanodevices as disclosed in Caruge, J. M., Halpert, J. E., Wood, V., Bulovic, V. & Bawendi, M. G. Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers. Nat. Photonics 2, 247-250 (2008) [13]. Interestingly, 2D semiconductor crystals, so called quantum wells, have been synthesized only by epitaxial growth on substrate using MBE for example.
The synthesis of 2D colloidal nanocrystals, nanoplatelets or nanodisks, is limited to few examples of metal as disclosed in Puntes, V. F. et al., A.P. Synthesis of hep-Co nanodisks. J Am Chem Soc 124, 12874-12880 (2002) [14] and in Xu, R. et al., Y.D. Single-crystal metal nanoplatelets: Cobalt, nickel, copper, and silver. Cryst. Growth Des. 7, 1904-1911 (2007) [15] and lanthanide-oxides as disclosed in Si, R., Zhang, Y. W., You, L. P. & Yan, C. H. Rare-earth oxide nanopolyhedra, nanoplates, and nanodisks. Angew Chemlnt Edit 44, 3256-3260 (2005) [16] materials as well as GuS as disclosed in Sigman, M. B. et al. Solventless synthesis of monodisperse Cu2S nanorods, nanodisks, and nanoplatelets. J Am Chem Soc 125, 16050-16057 (2003) [17] and NiS as disclosed in Ghezelbash, A., Sigman, M. B. & Korgel, B. A. Solventless synthesis of nickel sulfide nanorods and triangular nanoprisms. Nano Letters 4, 537-542 (2004) [18].
The general synthesis principle of monodisperse colloidal nanocrystals is based on the separation of the nucleation and growth stages. When the seed growth is favoured in 1 direction, nanorods are obtained, and 2D crystals are formed when growth is blocked in 1 direction.
While on the paper, things look simple, practically, the synthesis of colloidal nanocrystals of any dimensionality <3 relies on subtle combination of temperature, type and concentration of precursors and ligands (or surfactant), and cannot yet be guided by a precise understanding of the fabrication process at the molecular level.
The major advances in the synthesis of colloidal semiconductor nanocrystals were obtained with CdSe, first in 1993 with the synthesis of quantum dots as disclosed in reference [4], and then in 2000 with the synthesis of nanorods as disclosed in reference [6].
But the processes disclosed in the prior art are very expensive, difficult to be carried out, do not allow an easy synthesis of the colloidal material and do not offer any possibility to obtain a controlled homogenous and reproducible thicknesses of the materials. Further, the structures of the materials obtained with the processes of the prior art are only basic ones (1, 2D), with hazardous and heterogeneous thicknesses and very irregular lateral dimensions (3D).
No prior art successfully addresses these problems.
Thus, there remains a major need of a process that satisfactorily resolves these problems and disadvantages of the prior art.